1. Field of the Invention
The present invention relates to a light-emitting device in which a layer including an organic compound serves as a light-emitting layer and a method for manufacturing the light-emitting device.
2. Description of the Related Art
A light-emitting element containing an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at a low voltage, is expected to be applied to a next-generation flat panel display or a next-generation lighting device.
A light-emitting mechanism of a light-emitting element is as follows: a voltage is applied between a pair of electrodes where a layer containing an organic compound is interposed, electrons injected from a cathode and holes injected from an anode are recombined with each other at an emitting center of the layer containing an organic compound (also referred to as an EL layer) to form molecular excitons, and the molecular excitons release energy in returning to a ground state to emit light. Singlet excitation and triplet excitation are known as excited states, and light emission can probably be achieved through either of the excited states.
An EL layer has a structure typified by a stacked structure of a hole-transport layer, a light-emitting layer, and an electron-transport layer. EL materials for forming EL layers are roughly classified into low molecular (monomer) materials and high (polymer) molecular materials. An evaporation apparatus is used for film formation of the low molecular material.
In addition, a light-emitting element including a cathode, an EL layer, and an anode is called an EL element. There are two types of the EL elements: a type where an EL layer is formed between two kinds of stripe electrodes that are provided crosswise (simple matrix type); and a type where an EL layer is formed between pixel electrodes that are connected to TFTs and arranged in matrix and counter electrodes (active matrix type). However, when the pixel density is increased, the active matrix type where each pixel (or each dot) is provided with a switch is considered to be advantageous because it can be driven at a lower voltage.
An EL material forming an EL layer deteriorates very easily and deteriorates by easily oxidizing or absorbing moisture under the presence of oxygen or water. Therefore, there are problems in that the luminance of light emission and the lifetime of a light-emitting element decrease. Therefore, by covering of a light-emitting element with a sealing can, enclosure of dry air inside the sealing can, and attachment of a drying agent, oxygen or moisture is prevented from reaching the light-emitting element.
Further, a structure in which a spacer is provided over an insulating layer serving as a partition covering the periphery of a pixel electrode layer is disclosed in Patent Document 1.